In recent years, the energy density of lithium-ion batteries continues to improve, the traditional lithium cobalt oxide material has been unable to meet the needs of high specific energy batteries, higher capacity NCA and NMC materials began to climb the stage of history, especially the high-nickel NCA and NMC materials Capacity is up to 200mAh / g or so, with high capacity silicon anode material can increase the specific energy of lithium-ion battery above 250Wh / kg, and even up to 300Wh / kg.But high nickel NCA and NMC materials in increasing capacity There is virtually no potential, so in order to further improve the specific energy of lithium-ion batteries, we need to develop new high-capacity cathode materials among the many candidates, lithium-rich material is the one that seems to be the most promising The specific capacity of lithium-rich materials can reach over 300mAh / g, which is much higher than the current high-nickel materials, which brings boundless hope for the development of high-energy lithium-ion batteries, but enjoying the benefits of lithium-rich materials At the same time, we also have to face its problems, the first is the irreversible capacity, mainly because of the first charge and discharge process, oxygen reduction caused by O loss. Followed by the Voltage drop during the decay, which should be attributed to lithium rich material because O irreversible loss due to the impedance of the particle surface of the material due to structural changes in the cycle of increase.
The key to improve the performance of lithium-rich materials is to improve the stability of crystal structure and reduce the loss of oxygen, the current research to increase lithium-rich materials are also more for this.Common methods are elemental doping, such as South Jeonnam National University Paulraj Arunkumar significantly improves the cycle performance and rate performance of the material by incorporating Co3 + in Li2RuO3 In addition, surface coating is also a common method to improve the performance of lithium-rich materials such as Cheng Chen at Harbin Institute of Technology by Li1.2Mn0 .54Co0.13Ni0.13O2 particles coated with a layer of SnO2 on the surface, not only increased the material significantly improve the rate performance and cycle performance, SnO2 oxygen vacancy also increased the first capacity of lithium-rich materials. Today we want Introducing the research from Jinhyuk Lee of the University of California, Berkeley, Jinhyuk Lee, etc. In order to solve the problem of oxygen precipitation of the lithium-rich material, a small amount of F element is added to the material to reduce the oxygen precipitation and increase the Ni content Enhance the material's energy density, voltage platform and rate performance.
Lithium-rich material In order to ensure the structural stability of the material in the process of delithiation, generally we will over-material Li, such as Li1.211Mo0.467Cr0.3O2, Li1.3Mn0.4Nb0.3O2, Li1.2Mn0.4Ti0.4O2 , Which reduces the content of transition metal elements in the lithium-rich material and also makes the valence state of the transition metal element higher, which affects the capacity of the lithium-rich material, so the capacity of the lithium-rich material to play its role in the first charging A large part is due to the reduction of oxygen, which is also an important reason for the irreversible capacity of lithium-rich materials.
In order to solve the problem of oxygen precipitation, Jinhyuk Lee added a small amount of F element instead of some O element (LNF15) to Li1.15Ni0.375Ti0.375Mo0.1O2 (LN15), which increased the content of Ni2 + element in LN15 from 0.375 to 0.45, So that the material capacity is more dependent on the redox process of Ni element, rather than the irreversible O element reduction, so the reversible capacity of the material also increased from 225mAh / g to 266mAh / g, the energy density increased to 790Wh / kg. 3300Wh / l.
The following figure shows the XRD and elemental distribution of Li-rich materials synthesized by solid-phase method by Jinhyuk Lee. From the figure, we can see that there is a slight increase of unit cell size with the increase of lithium excess, such as LN15 An excess of 15%) had a unit cell size of 4.1444A, but the unit cell size of LN20 (lithium overbased 20%) increased 4.1449A but the unit cell size of LNF15 decreased to 4.1415A after the addition of the F unit in LN15, whereas F The study of elemental distribution shows that the F element does not form a new phase in the lithium-rich material but is uniformly distributed inside the lithium-rich material.
The addition of F element can well stabilize the crystal structure of the material during charging and discharging. As shown in the following figure, the materials of LN15 and LN20 (Li1.2Ni0.333Ti0.333Mo0.133O2) all appear in a voltage platform around 2.2V, corresponding to The reduction of Ti4 + and Mo6 + on the particle surface occurred only after some oxygen was lost, but the 2.2V voltage platform disappeared significantly after the addition of F, while LNF15 and S-LNF15 (using a vibrating mill Mixed) also significantly higher than the non-F LN15 material, 210mAh / g and 250mAh / g respectively.
In order to confirm the above results, Jinhyuk Lee studied the behavior of lithium-rich materials releasing O during charging as shown in the following figure. It can be seen from the figure that LN15 and LN20 materials all start to produce O2 around 4.35V, but adding F The post-elemental LNF15 produced an increase of O2 voltage to 4.5 V. The O2 production also dropped significantly, with the quantities of O produced by LN15 and LN20 being 0.26 and 0.40 μmol / mg, respectively, over the entire charging period, but the LNF15 production dropped to 0.07umol / mg, indicating that the F element is still very significant in stabilizing the structure of the lithium-rich material.
The addition of F element not only improves the stability of the material, but also increases the ionic conductivity of the material and reduces the polarization of the material during charging and discharging. For example, the GITT, the voltage of the LNF 15 material in the galvanostatic batch titration in FIG. Hysteresis is significantly lower than the LN20 material, indicating that the material diffusion coefficient was significantly higher than the material without the addition of F elements.
Jinhyuk Lee improves the reversible capacity of the material by adding F element to the lithium-rich material, which can further reduce the ion diffusion resistance of the material and reduce the charge-discharge process The polarization of the battery further provides the energy density of the material.Minhyuk Lee's research provides an effective solution to the problems of high irreversible capacity and poor rate performance of lithium-rich materials, and its use of solid-phase method is also very suitable for practical Production applications.
